Optical waveguides can be used to expand or replicate the exit pupil of an imaging system in one or two dimensions. Typically, light from the exit pupil of the imaging system is received in the waveguide through an in-coupling, and travels through the waveguide in a direction. The light then exits the waveguide through an out-coupling that is larger than the in-coupling, creating an expanded pupil.
Current waveguides typically use optical elements such as crossed gratings and double-sided gratings to in-couple and out-couple light. Crossed gratings are gratings that are periodic in two dimensions, while double-sided gratings are gratings that are located on both surfaces of the optical waveguide.
However, there are drawbacks associated with both cross and double-sided gratings. Crossed gratings are difficult to manufacture and typically do not have as many degrees of freedom (e.g., depth, slant, duty cycle, and fill factor) as double-sided gratings. Double-sided gratings have tight rotation tolerances and are therefore also difficult to manufacture.